Examples of such devices are described in our previous patent applications EP-A-0726226 and EP-A-0950431.
In each example, across one of the opposed surfaces of the membrane, air is passed along a flow path, while a current is applied across the tri-layer, and oxygen passes through the membrane to a gas flow path at the other of the opposed membrane surfaces, whilst oxygen depleted air, which is rich in nitrogen, passes along the one opposed surface, and hence from the device.
In EP-0726226, a cross flow device is described, with air being fed across the one opposed membrane surface from a first side of the device, and the oxygen depleted gas passing to an opposite second side of the device, whilst the oxygen flows crosswise across the other opposed surface of the membrane to a third side of the device intermediate the first and second sides.
In EP-A-0950431, gas flow paths are provided through a stack of the tri-layers, in flow paths which include passages which extend through the tri-layers.
Such ionic conduction devices may be used alternatively as fuel cells, with a fuel gas such as hydrogen, being combined with oxygen along one opposed surface of the membrane, and exhaust gas being collected from the other opposed surface of the membrane, the device generating electrical energy across the tri-layer.
Particularly where the ionic conduction device is used to separate gases from a gas mixture, e.g. oxygen from air, such devices tend to operate inefficiently. This is because it is difficult to maintain a thermal equilibrium in the device as at least after a warm-up period, as electrical energy supplied is converted into heat which must be removed. Conventionally, the airflow to the device is optimized to achieve a thermal equilibrium, but this results in the oxygen depleted gas still containing significant volumes of oxygen, as the airflow is too great for all the oxygen to pass through the membrane as the air flows. If the airflow is reduced, more oxygen is separated from the air, but the device may become overheated.
It is particularly important where it is desired for the oxygen depleted air to be used, for example as an inert gas in an aircraft for example, in the aircraft's fuel tanks to reduce any risk of fire/explosion, for the oxygen depleted gas to have as low as possible an oxygen content.